Displacement transducers are used to measure small displacements typically on the order of a fraction of a millimeter. Naturally, with measurements of this magnitude error must be minimized. One source of error is human error which comes about when standard dials and guages are read to determine displacement. It is desirable to have a digital guage which would minimize the possibility of human error.
One type of transducer is known as a vibrating string transducer which utilizes the physics law of vibrating strings: EQU f=1/2L.sqroot.q/s
where
f=frequency of vibration of the wire PA1 L=length of wire suspended between two points PA1 q=tensile stress in the wire PA1 s=density of the wire
Thus, by measuring the frequency of vibration of the wire, its length can be determined. Any change of the length causes a corresponding change of the natural oscillation frequency of the wire. The mechanical oscillation of the wire is converted into an electric signal by means of an inductive pickup system located near the oscillating wire as taught by Kurt S. Lion at pages 8385 in his book Instrumentation in Scientific Reasearch; Electrical Input Transducers, published in 1959 by McGrawHill Book Company, Inc. Lion mentions that the steel wire or string, in vibrating, varies the magnetic flux in the air gap of the pickup system. The output signal is amplified and fed back into an electromagnet to keep the string excited at its natural frequency.
U.S. Pat. Nos. 2,929,196, 2,949,727 and 2,960,817 granted to Max Hetzel on Mar. 22, 1960, Aug. 23, 1960, Nov. 22, 1960, respectively, disclose a watch mechanism. U.S. Pat. No. 2,960,817 is exemplary of these patents and discloses a two-tined tuning fork, one tine of which operates a ratchet wheel which indexes the timepiece. A circuit similar to the Lion circuit above is used to excite the tines which then vibrate at a natural frequency determined by their length. However, the length of the tuning fork is fixed so that it oscillates at a predetermined frequency thus making this fixed movement unsuitable for measuring displacement. It is therefore desirable to have a displacement transducer which utilizes an accurate vibrating device and produces a digital output which can be interfaced with other digital equipment.